A method of the type mentioned above is known from the market. It is used in internal combustion engines having direct fuel injection. In such internal combustion engines a NOx adsorption catalyst is provided to reduce emissions. The internal combustion engine usually operates in an operating mode in which the fuel-air mixture present in the combustion chamber is lean. The nitrogen oxides produced in this operating mode are absorbed by the NOx adsorption catalyst and stored temporarily.
However, for the operation of the internal combustion engine having the NOx adsorption catalyst it is necessary from time to time to switch from this first “lean” operating mode to a second “rich” operating mode and vice versa. In the rich operating mode, the NOx adsorption catalyst is purged of nitrogen oxides stored therein as completely as possible. Other known methods, in particular in the control of diesel engines, use the fuel quantity to be injected as the central variable in the control of the engine torque. In doing so, the setpoint torque, determined on the basis of a driver-desired torque and coordinated with other demands, such as a transmission control, is converted into the fuel quantity to be injected by means of a special conversion module. The fuel quantity is determined by a characteristic curve, which is a function of the setpoint torque, using the instantaneous speed as characteristic-curve parameter (characteristic set).
In German Patent Application No. DE 100 30 936, a method is described in which a setpoint air mass and a setpoint fuel mass are determined from the input variables, fuel mass in lean operation, air mass in lean operation, lambda advantageous for regeneration and actual air mass. For this purpose, lambda values are converted into efficiency values, and vice versa, in three different processing blocks.
However, this method is not suitable for converting the setpoint torque into the fuel quantity to be injected when the internal combustion engine has a high excess of air (lean operation) since the torque and the fuel quantity are not linked in a simple manner in this operating mode.
It is an objective of the present invention to further refine a method of the type mentioned above, in such a way that it may be used in as uncomplicated a manner as possible and the two actuating variables, fuel quantity and air mass, are ascertained in a physically correct manner even when the air-fuel ratio is selectively modified.